The preparation of DAC by oxidative direct carbonylation of aromatic hydroxy compounds in the presence of CO, O2 and a noble metal catalyst is known (see e.g. DE-OS 27 38 437, U.S. Pat. No. 4,349,485, U.S. Pat. No. 5,231,210, EP-A 667 336, EP-A 858 991, U.S. Pat. No. 5,760,272). Palladium is preferably employed as the noble metal. A cocatalyst (e.g. salts of manganese or cobalt), a base, sources of bromide, quaternary salts, various quinones or hydroquinones and drying agents can additionally be employed. The process can be carried out in a solvent.
However, the known processes do not give yields which are satisfactory for an industrial reaction and produce relatively large amounts of by-products. It is therefore desirable to provide a process which is optimized in respect of yield and product quality.
Since these are processes which are to provide a product on a large industrial scale, small optimizations already mean a great improvement.